This invention relates to improvements in electrical power assisted steering systems for vehicles of the kind in which an electric motor is operatively connected to a steering assembly via a gearbox to apply an assistance torque to the steering assembly, and in particular to apparatus for determining the angular position of a portion of a steering assembly.
It is well known to provide electric power assisted steering (EPAS) systems of the kind set forth. The steering assembly typically comprises a hand wheel connected to a steering shaft which is operatively connected to one or more roadwheels through a steering rack, although many different assemblies are in common use. Some EPAS systems use brushless motors in which the electric motor is provided with a motor position sensor to control the timing of switching, or communication of windings of the motor. The motor position sensor typically comprises an electromagnetic type switch or switches which changes state whenever a magnet provided on the rotor passes the sensor. Alternatively, a magnetised disc can be mounted on the rotor shaft and the sensor may detect movement of the magnets on the disc.
For example, in a 3-phase brushless permanent magnet motor, three Hall effect sensors can be located around the rotor in such a manner that a crude measurement of rotor electrical position can be obtained. However, this is unsuitable for use as an indication of the position of the steering shaft since the output will repeat within a single turn of the motor shaft and thus produce an ambiguous signal. The output also depends on the ratio of the gearbox.
In order to provide an accurate measurement of steering shaft position, it is known to provide an angular position sensor either mounted directly on the steering shaft or connected thereto via a gear drive. This produces an output which does not depend on the gearbox ratio as it reads directly from the steering shaft, but is expensive to produce.
In accordance with a first aspect, the invention provides an electric power assisted steering system of the kind in which an electric motor is operatively connected to a steering assembly through a gearbox to apply an assistance torque to the steering assembly, and comprising: a motor position sensing means adapted to produce an output indicative of the angular position of the motor rotor; and characterised by further comprising counting means adapted to count transitions in the output of the motor position sensing means to produce a count signal indicative of the angular position of the rotor relative to an arbitrary zero position; a steering position sensing means adapted to produce at least one position index signal indicative of a known angular position of a portion of the steering assembly and reset means adapted to reset the count signal produced by said counting means whenever said portion of said steering assembly is at said known angular position by monitoring said index signal.
The invention thus provides apparatus for monitoring the angular position of the steering assembly from the output of the motor position sensing means by counting transitions in the output of the motor position sensing means, and resetting the count whenever an index signal is produced from a sensor which indicates that the steering assembly position corresponds to the straight ahead position for the vehicle. The index signal may be provided from a steering column sensor, directly from a sensor adapted to monitor the position of a steering rack of the steering assembly, or from a yaw rate sensor adapted to produce an output indicative of the yaw rate of the vehicle.
Thus, a low cost index position sensor can be provided on the steering shaft simply to facilitate the reset timing.
By counting transitions we mean, for example, incrementing the count signal when the sensing means output changes state corresponding to rotation in one direction, and decrementing the count signal when a change of state occurs corresponding to rotation in the opposite direction. The value of the count signal is therefore indicative of the angular position of the steering shaft relative to the known angular position at which the reset occurs.
Preferably, the motor position sensing means may comprise one or more electromagnetic effect sensors adapted to detect the position of one or more rotor magnets or magnets fixed relative to the rotor of the motor. In an especially advantageous arrangement, the motor comprises a brushless permanent magnet motor, and the magnetic-effect sensors detect the position of the rotor magnets. Of course, other types of sensor could be used as an alternative.
The output signal from the motor position sensing means may, in addition to producing the count signal, advantageously be used to control the timing of, or commutation, of the motor rotor windings. The sensing means may comprise one or more Hall effect sensors. Preferably, it comprises three Hall effect sensors adapted to produce a three-bit output signal.
The system may include means adapted to xe2x80x9cstrobexe2x80x9d or periodically energise the motor position sensing means and/or the steering position sensing means and means adapted to sample the output of the sensing means when energised. The sensors may then be de-energised between samples. This minimises the average current drain compared to running the sensors continuously. It also enables the counter to track the steering shaft position even when the steering system is powered down by turning off the vehicle ignition without draining the vehicle battery excessively. A latch may be provided to latch the sampled value.
The steering position sensing means may be adapted to produce a single position index signal which may correspond to the straight ahead position of the steering shaft, or any other preferred angle of the steering. For instance, a pulse may be produced as the steering shaft rotates past a known position. It may comprise a single magnetic effect sensor adapted to detect the passing of a magnet on the shaft. Thus, the output of the steering position sensing means may have a first output value for all positions of the shaft other than at the index position when it take a second value. However, any other suitable output may be used as long as the index position can be identified by processing the output.
If one index signal is produced per revolution, the counter may be reset to zero (or another preferred value) once every time the steering shaft revolves. In this case, the maximum value held in the counter will depend on the number of transitions of the output of the motor sensing means for a single turn of the motor and upon the number of turns of the motor for a single turn of the steering shaft. This is obviously dependent on the gearbox ratio. Nevertheless, since the gearbox ratio remains constant, the value held in the counter will provide an accurate indication of the shaft position within a revolution of the steering shaft. Resetting helps prevent the total count drifting off over time. This may, for instance, occur if false xe2x80x9ctransitionsxe2x80x9d are recorded.
Preferably, however, the index signal is produced only when the steering is in the straight ahead position or some angle associated with the position, so that the value held in the counter may represent absolute steering angle relative to this straight ahead position.
The counting means may comprise a 16-bit counter although smaller or larger counters can be used. They may be binary counters.
The system may be battery operated, or at least partially battery operated. Battery status means may be provided which is adapted to detect when the battery is disconnected from the system. An error flag may be raised when the battery status means has detected removal or disconnection of the battery. If this flag is raised, the counter value may be treated as erroneous or unreliable on a subsequent power up, (i.e. reconnection of the battery). This is because the motor rotor may have rotated without the counter being updated is if was not powered up.
A processing means may be provided to correlate the value stored in the counter with a measure of the steering shaft position from a knowledge of the ratio of the gearbox.